The present invention relates to radiation shields, and in particular to such shields for protecting a tool from being damaged by high-irradiance radiation when processing a workpiece with the tool.
In many modern-day manufacturing applications, such as semiconductor manufacturing or materials processing, it is desirable to process an object (xe2x80x9cworkpiecexe2x80x9d), with a beam of high-irradiance radiation to modify the chemical, physical or electrical properties of the workpiece. In semiconductor manufacturing, the substrate is often a silicon wafer, and in materials processing, the substrate can be a metal plate. For most applications, the radiation source is a high-irradiance (units: J/cm2) laser. Efficient manufacturing techniques typically require that a robotically controlled tool deliver this high-irradiance radiation to the workpiece, preferably with little user intervention.
With reference to FIG. 1, prior art processing tool 10 comprises, in order along axis A1, a workpiece support member 14 (i.e., a xe2x80x9cchuckxe2x80x9d) and a tool portion 20. The latter includes a variety of components such as vacuum lines, electrical cables, mechanical apparatus, metal surfaces, and the like. Workpiece support member 14 supports a workpiece 24 having an upper surface 26, a lower surface 28 and an outer edge 30. A source of radiation (not shown) irradiates upper surface 26 of workpiece 24 with high-irradiance radiation 34.
A problem often encountered in radiation-based processing tools such as processing tool 10 is that a portion 38 of high-irradiance radiation 34 incident workpiece 24 spills over onto tool portion 20, or even onto workpiece support member 14. Portion 38 is referred to herein as xe2x80x9cspillover radiation.xe2x80x9d This happens most often when a region on surface 26 near the edge of workpiece 24 is being processed. Since spillover radiation 38 has sufficiently high irradiance to modify the surface properties of workpiece 24, it also typically has sufficient irradiance to modify the surface properties of tool portion 20. In time, spillover radiation 38 can damage tool portion 20, can cause unwanted heating problems within processing tool 10, and can result in potentially harmful reflections.
Unfortunately, it is not possible to simply shield the tool from spillover radiation using conventional shields made of metal or plastic, because the high irradiance beam would damage such a shield. For instance, simply extending workpiece support member 14 to capture spillover radiation is not a practical solution because the workpiece support member typically needs to be made of a light, machinable metal such as aluminum, which is susceptible to damage from high-irradiance radiation.
Ideally, it is desirable to intercept spillover radiation 38 with a shield capable of rendering the radiation harmless before it irradiates the tool. If, for example, a metallic shield is placed between workpiece 24 and tool portion 20 to intercept spillover radiation 38, the metallic shield will, in all likelihood, ablate or be damaged by the radiation. Further, a metallic shield may reflect radiation onto other portions of processing tool 10. Generally, speaking, any shielding material that relies primarily upon surface absorption of radiation will probably be damaged.
There are several prior art shields designed to intercept radiation and render it harmless. For example, U.S. Pat. No. 5,153,425, entitled xe2x80x9cBroadband Optical Limiter with Sacrificial Mirror to Prevent Irradiation of a Sensor System by High Intensity Laser Radiation,xe2x80x9d describes a broadband optical limiter for use in combination with a sensor system operative to prevent irradiation of the sensor system by laser radiation of unknown wavelengths having intensity levels sufficient to damage or disable the sensor system. The broadband optical limiter is further operative to throughput, with minimal optical distortion at wide-angle fields of view, electromagnetic radiation in the operating spectral band(s) of the sensor system. The broadband optical limiter includes a flat or spherically shaped sacrificial mirror that is operative to reflect electromagnetic radiation in the operating spectral band(s) of the sensor system and to be optically machined, i.e., vaporized, by focused laser radiation of unknown wavelengths having intensity levels sufficient to damage or disable the sensor system to create reflective dead spot. The reflective dead spot prevents the focused laser radiation from being throughputted to the sensor system. The broadband optical limiter further includes optical components to focus incident electromagnetic and laser radiation onto the sacrificial mirror, to turn incident electromagnetic and laser radiation out of the field of view of the sensor system, and to turn electromagnetic radiation reflected by the sacrificial mirror back into the field of view of the sensor system. A major disadvantage of this type of shield, however, is that it is sacrificial, and changes due to the irradiation. Such shields tend to need to be replaced frequently.
U.S. Pat. No. 4,114,985, entitled xe2x80x9cShield for High Power Infrared Laser Beam,xe2x80x9d describes shielding from and the termination of high power infrared laser beams by interception of the beam by one of two spaced, juxtaposed, ceramic (i.e., clay-based) sheet members. The beam-intercepting member has a thickness to beam power density relationship that allows opaque to translucent conversion of the portion thereof illuminated by the beam. The translucent portion subsequently diffuses the beam. The second ceramic sheet member then absorbs the diffused beam. A major shortcoming of this shield, however, is that it requires two clay-based, opaque sheet members, with the first sheet having to be of sufficient strength to cause a transformation from opaque to translucent by virtue of the incident radiation.
U.S. Pat. No. 4,575,610, entitled xe2x80x9cLaser Shielding Device,xe2x80x9d describes a laser-shielding device having two spaced-apart layers of shielding material defining a sealed chamber between the two layers. At least one layer degrades in the presence of an impinging laser beam, creating a hole through the layer. A pressure change in the chamber is sensed and signaled to a machine controller to stop the lasing operation. Unfortunately, this shield device is not well-suited for protecting a tool from spillover radiation, since the shield is sacrificial and thus would need to be replaced often. Further, the shield is pressurized, which adds to its complexity.
The present invention relates to radiation shields, and in particular such shields for protecting a tool from being damaged by high-irradiance radiation when processing a workpiece with the tool using high-irradiance radiation.
A first aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, an absorption coefficient, a volume and a thickness, and designed to absorb in said volume, a portion of said high-irradiance radiation that would otherwise be incident the tool portion, wherein the shield maintains said absorbed high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.
A second aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, a scattering coefficient, a volume and a thickness, and designed to scatter in said volume a portion of said high-irradiance radiation that would otherwise be incident said tool portion, wherein said shield maintains said scattered high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.
A third aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, an absorption coefficient, a volume, a scattering coefficient and a thickness, wherein said shield is designed to absorb and scatter in said volume a portion of the high-irradiance irradiation, wherein the shield maintains said absorbed high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.
A fourth aspect of the invention is an apparatus that prevents a tool portion having an irradiance damage threshold from being irradiated by high-irradiance radiation from a light source while irradiating a workpiece. The apparatus comprises a workpiece support member for supporting a workpiece, with one of the shields described immediately above, arranged between the light source and the tool portion.
A fifth aspect of the invention is a method of processing a workpiece with high irradiance radiation from a light source using a workpiece processing tool having a tool portion with an irradiance damage threshold. The method comprises the steps of first, supporting the workpiece on a workpiece support member, then arranging one of the shields described immediately above between the light source and workpiece so as to intercept any of the high-irradiance radiation that would otherwise be incident the tool portion, and then irradiating the workpiece.